This invention relates generally to expansion valves used in refrigeration systems and particularly to an expansion valve that provides for additional flow of refrigerant during pulldown conditions.
In any air conditioning system or refrigerated system, such as a display case, walk in room, freezer or chiller, the load on the evaporator is always greatest during pulldown conditions. The pulldown conditions are experienced, by way of example, when a display case has been defrosted or when the case has been loaded with a relatively warm food product. Once the initial pulldown period is over, and the discharge air from the evaporator is normal for the particular product being conditioned, the load on the evaporator is much smaller than during pulldown.
In practice, the pulldown load can be as much as 3 to 3.5 times greater than normal load. In consequence, when sizing a thermostatic expansion valve in the past, for example for a display case, a compromise was found necessary so that the valve was sized to provide a pulldown period as short as possible, the result of which was an unreasonably oversized valve for normal holding loads. Oversized valves typically result in control problems and affect the efficiency of the refrigeration system.
Pulldown can also occur in an air conditioning system where the conditioned space is not controlled and allowed to approach outside ambient temperature. In the past, particularly in large systems, unloading features in the compressor were often used as necessary to accommodate capacity differences.
This improved expansion valve overcomes these and other problems in a manner not revealed by the known prior art.